The respiratory or acoustic impedance of a human respiratory system can be measured to obtain information concerning the resistance, compliance and inertia of the airways, lungs and chest wall. This information is useful in diagnosing the nature and severity of a variety of respiratory diseases such as chronic obstructive pulmonary disease (COPD), asthma and bronchitis.
In a forced oscillation technique (FOT), such as that described in“Expiratory Flow Limitation Detected by Forced Oscillation and Negative Expiratory Pressure” by Dellacà et al., pages 363-374 European Respiratory Journal Vol. 29 No. 2 (reference 1), acoustic waves are directed into the respiratory system while the person breathes normally, and the response is measured to determine the respiratory impedance.
The respiratory impedance describes the frequency-dependent relation in the oscillations resulting from the acoustic waves in terms of flow and pressure. Where the respiratory impedance varies from inspiration to expiration (as in some diseases and other medical conditions), the respiratory impedance must be estimated with a fine time resolution. However, in the prior art, little consideration has been given to the reliability of techniques for estimating the respiratory impedance in time intervals that are short enough for physiological purposes (i.e. shorter than the duration of an inhalation or exhalation).
Therefore, there is a need for a method and apparatus for reliably estimating the respiratory impedance in short time intervals.